How to Calculate Turning Radius

by Richard Rowe

"Turning radius" is one of those terms that has kind of taken on a life of its own, mostly through public misunderstanding. Technically, a "radius" is half the diameter of a whole circle; interesting information in terms of comparing how well two cars can do a U-turn, but not exactly useful in the objective sense. Most magazines have long since given up using "turning radius" in the colloquial sense in favor of "turning circle" to describe the space a vehicle needs to do a U-turn. But, one is easy to derive from the other, if you're feeling particularly technical.

1

Find a flat parking lot with plenty of open space, and park with your passenger-side door near the edge of the lot. Count on needing 50 feet or so of open space to the left.

2

Have your assistant mark the ground with chalk about an inch to the right of where your right-front tire touches the ground. This bit of extra clearance is necessary, because the tire's sidewall will typically bulge out past the tread.

3

Start the engine, and turn your steering wheel all the way to the left, as far as it will go. Put the vehicle in "drive," and very gently, very slowly let your foot off the brake. Proceed in a circle at idle speed.

4

Stop just before you've reached halfway around the circle -- call it 170 out of 180 degrees. Have your assistant mark the ground next to your right-front tire.

5

Proceed forward another six inches or so, and have your assistant place another mark. Keep stopping and marking the ground every six inches or so until you're pointed back toward the edge of the parking lot.

6

Move the vehicle and park it. Measure from your first reference mark, where you started, to the furthest out of your other reference marks. The distance from the first mark to the furthest mark is your turning circle, or the space your vehicle needs to do a U-turn without hitting a curb.

7

Divide this measurement by two to get your "turning radius." This information is almost useless in the real world, but it's nice to say you have it.

Tips

checkMany new vehicles use speed-sensitive power steering, which changes the steering response according to how fast you're going. Vehicles with these systems may have a smaller turning circle at 5 mph than they do at 15 mph; just something to bear in mind if you have a habit of whipping around at stop lights without slowing to idle speed.

checkIn the United States, lanes can be as wide as 12 feet, or as narrow as 9 feet. The average median between lanes on a divided highway is 3 to 5 feet. So, a U-turn from the left turn lane on a four-lane, divided highway requires a turning circle of about 30 to 40 feet -- well within the range of most passenger cars. However, most of the time, only the smallest cars with the shortest wheelbases can manage a U-turn on an 18-foot-wide, two-lane backroad without going off one side or the other.

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About the Author

Richard Rowe has been writing professionally since 2007, specializing in automotive topics. He has worked as a tractor-trailer driver and mechanic, a rigger at a fire engine factory and as a race-car driver and builder. Rowe studied engineering, philosophy and American literature at Central Florida Community College.